What does ADD DRAG mean?

This message appears on the left side of the ND in the following
situations and is prompting the pilot to extend the speedbrakes:

The aircraft is unable to meet the next altitude or speed
constraint.

The aircraft's speed in descent has risen to 10 knots or
greater above the commanded speed due to situations like improper winds
entry or a PROF path that is too steep to maintain the target airspeed.
The message will clear when the aircraft is within 3 knots of the
target.

This is essentially the FMS asking assistance from the pilot to
reduce speed, but it is not always obligatory to do so. If you see that
the speed is reducing and it will eventualy get back on target you don't necessarily
have to extend speedbrakes. For example, if an abrupt wind
change makes the speed go temporarily out of target, it is up to you to
decide if and how much (1/3, 2/3, FULL) speedbrakes should be applied.
Remember to retract the speedbrakes when the message goes away. It is
easy to forget them because the MD-11 displays do not provide any
indication of the speedbrake position.